[fir2ir] Treat code fragments returning 'Nothing' as non-void

By default, the JVM backend maps the 'Nothing' return type to 'Unit'
(see 'JavaToKotlinClassMap.mapKotlinToJava'). For code fragments, such
behavior is rather unexpected, especially if the 'Nothing' type appeared
as a result of a smart cast.

The change replaces the return type of 'Nothing'-returning fragments
to 'Any?' so it's mapped to debugger-friendly 'Object' instead.

^KT-70860 Fixed

Author: yanex

analysis/analysis-api/testData/components/compilerFacility/compilation/codeFragments/resultNothing.ir.txt

@@ -5,7 +5,7 @@ MODULE_FRAGMENT
       CONSTRUCTOR visibility:public returnType:<root>.CodeFragment [primary]
         BLOCK_BODY
           DELEGATING_CONSTRUCTOR_CALL 'public constructor <init> () [primary] declared in kotlin.Any'
-      FUN name:run visibility:public modality:FINAL returnType:kotlin.Nothing
+      FUN name:run visibility:public modality:FINAL returnType:kotlin.Any?
         EXPRESSION_BODY
           BLOCK type=kotlin.Unit origin=null
             CALL 'public final fun call (): kotlin.Nothing declared in <root>.ContextKt' type=kotlin.Nothing origin=null

analysis/analysis-api/testData/components/compilerFacility/compilation/codeFragments/resultNothing.txt

@@ -18,8 +18,8 @@ public final class CodeFragment {
     MAXLOCALS = 1
 
   // access flags 0x19
-  public final static run()Ljava/lang/Void;
-  @Lorg/jetbrains/annotations/NotNull;() // invisible
+  public final static run()Ljava/lang/Object;
+  @Lorg/jetbrains/annotations/Nullable;() // invisible
    L0
     LINENUMBER 24 L0
     INVOKESTATIC ContextKt.call ()Ljava/lang/Void;

analysis/analysis-api/testData/components/compilerFacility/compilation/codeFragments/resultNothingBeforeSmartCast.ir.txt

@@ -5,7 +5,7 @@ MODULE_FRAGMENT
       CONSTRUCTOR visibility:public returnType:<root>.CodeFragment [primary]
         BLOCK_BODY
           DELEGATING_CONSTRUCTOR_CALL 'public constructor <init> () [primary] declared in kotlin.Any'
-      FUN name:run visibility:public modality:FINAL returnType:kotlin.Nothing?
+      FUN name:run visibility:public modality:FINAL returnType:kotlin.Any?
         VALUE_PARAMETER kind:Regular name:p0 index:0 type:kotlin.String?
         EXPRESSION_BODY
           BLOCK type=kotlin.Nothing? origin=null

analysis/analysis-api/testData/components/compilerFacility/compilation/codeFragments/resultNothingBeforeSmartCast.txt

@@ -18,14 +18,13 @@ public final class CodeFragment {
     MAXLOCALS = 1
 
   // access flags 0x19
-  public final static run(Ljava/lang/String;)Ljava/lang/Void;
+  public final static run(Ljava/lang/String;)Ljava/lang/Object;
   @Lorg/jetbrains/annotations/Nullable;() // invisible
     // annotable parameter count: 1 (invisible)
     @Lorg/jetbrains/annotations/Nullable;() // invisible, parameter 0
    L0
     LINENUMBER 21 L0
     ALOAD 0
-    CHECKCAST java/lang/Void
     ARETURN
    L1
     LOCALVARIABLE p0 Ljava/lang/String; L0 L1 0

analysis/analysis-api/testData/components/compilerFacility/compilation/codeFragments/resultNothingSmartCast.ir.txt

@@ -5,7 +5,7 @@ MODULE_FRAGMENT
       CONSTRUCTOR visibility:public returnType:<root>.CodeFragment [primary]
         BLOCK_BODY
           DELEGATING_CONSTRUCTOR_CALL 'public constructor <init> () [primary] declared in kotlin.Any'
-      FUN name:run visibility:public modality:FINAL returnType:kotlin.Nothing?
+      FUN name:run visibility:public modality:FINAL returnType:kotlin.Any?
         VALUE_PARAMETER kind:Regular name:p0 index:0 type:kotlin.String?
         EXPRESSION_BODY
           BLOCK type=kotlin.Nothing? origin=null

analysis/analysis-api/testData/components/compilerFacility/compilation/codeFragments/resultNothingSmartCast.txt

@@ -18,14 +18,13 @@ public final class CodeFragment {
     MAXLOCALS = 1
 
   // access flags 0x19
-  public final static run(Ljava/lang/String;)Ljava/lang/Void;
+  public final static run(Ljava/lang/String;)Ljava/lang/Object;
   @Lorg/jetbrains/annotations/Nullable;() // invisible
     // annotable parameter count: 1 (invisible)
     @Lorg/jetbrains/annotations/Nullable;() // invisible, parameter 0
    L0
     LINENUMBER 21 L0
     ALOAD 0
-    CHECKCAST java/lang/Void
     ARETURN
    L1
     LOCALVARIABLE p0 Ljava/lang/String; L0 L1 0

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrConverter.kt

@@ -37,6 +37,7 @@ import org.jetbrains.kotlin.fir.java.javaElementFinder
 import org.jetbrains.kotlin.fir.references.toResolvedValueParameterSymbol
 import org.jetbrains.kotlin.fir.scopes.processAllFunctions
 import org.jetbrains.kotlin.fir.symbols.lazyDeclarationResolver
+import org.jetbrains.kotlin.fir.types.isNothingOrNullableNothing
 import org.jetbrains.kotlin.fir.types.resolvedType
 import org.jetbrains.kotlin.fir.types.toRegularClassSymbol
 import org.jetbrains.kotlin.ir.IrBuiltIns
@@ -58,6 +59,8 @@ import org.jetbrains.kotlin.ir.symbols.IrPropertySymbol
 import org.jetbrains.kotlin.ir.symbols.UnsafeDuringIrConstructionAPI
 import org.jetbrains.kotlin.ir.symbols.impl.IrConstructorSymbolImpl
 import org.jetbrains.kotlin.ir.symbols.impl.IrSimpleFunctionSymbolImpl
+import org.jetbrains.kotlin.ir.types.IrType
+import org.jetbrains.kotlin.ir.types.makeNullable
 import org.jetbrains.kotlin.ir.util.NaiveSourceBasedFileEntryImpl
 import org.jetbrains.kotlin.ir.util.defaultType
 import org.jetbrains.kotlin.ir.util.fileOrNull
@@ -307,17 +310,14 @@ class Fir2IrConverter(
         }
 
         IrSimpleFunctionSymbolImpl().let { irSymbol ->
-            val lastStatement = codeFragment.block.statements.lastOrNull()
-            val returnType = (lastStatement as? FirExpression)?.resolvedType?.toIrType() ?: builtins.unitType
-
             IrFactoryImpl.createSimpleFunction(
                 UNDEFINED_OFFSET, UNDEFINED_OFFSET,
                 IrDeclarationOrigin.DEFINED,
                 conversionData.methodName,
                 DescriptorVisibilities.PUBLIC,
                 isInline = false,
                 isExpect = false,
-                returnType,
+                computeCodeFragmentReturnType(codeFragment),
                 Modality.FINAL,
                 irSymbol,
                 isTailrec = false,
@@ -356,6 +356,21 @@ class Fir2IrConverter(
         return irClass
     }
 
+    private fun computeCodeFragmentReturnType(codeFragment: FirCodeFragment): IrType {
+        val lastStatement = codeFragment.block.statements.lastOrNull()
+        if (lastStatement is FirExpression) {
+            val lastStatementType = lastStatement.resolvedType
+            if (lastStatementType.isNothingOrNullableNothing) {
+                // Unless the last statement type is explicitly 'Unit', treat code fragments like they have a return value.
+                return builtins.anyType.makeNullable()
+            }
+
+            return lastStatementType.toIrType()
+        }
+
+        return builtins.unitType
+    }
+
     // `irClass` is a source class and definitely is not a lazy class
     @OptIn(UnsafeDuringIrConstructionAPI::class)
     private fun delegatedMembers(irClass: IrClass): List<FirDeclaration> {